Hong Kong residents huddled indoors Sunday as strong wind sent debris flying ahead of the arrival of Typhoon Mangkhut, once the world's strongest storm this year before it weakened after slamming into the Philippines.

The Hong Kong Observatory (HKO) raised the storm signal to T10 -- the highest level possible -- Sunday morning local time, as the storm continues to carve a destructive path from the Philippines toward mainland China, CNN reported.

Fierce winds have already torn off roofs, smashed windows and downed trees in Hong Kong, as authorities warned of the threat of storm surges and flooding from torrential rain. The city is almost entirely shut down.

Mangkhut was recorded packing sustained winds of 173 kilometers per hour (107 miles per hour) and guests up to 223 kilometers per hour (138 miles per hour) as the storm's eye passed south of the territory in the early afternoon, according to the HKO.

At 3:00 p.m., the storm was 110 kilometers (68 miles) southwest of Hong Kong, and heading for the surrounding Pearl River Delta, home to 120 million people, the HKO reported later Sunday.

The storm is expected to be one for Hong Kong's record books. It's only the 15th time in the last 60 years that a T10 has been hoisted; the last was for Super Typhoon Hato last year.

The mainland Chinese city of Guangzhou is also on high alert. The city issued its highest typhoon emergency alert, according People's Daily, a state-run media outlet. More than 100,000 people have been evacuated.

On Saturday, it plowed into the Philippines, flattening homes in small towns and villages on the northern island of Luzon.

CNN Philippines reported that 25 people had been killed since the typhoon made landfall in the early hours of Saturday, quoting Political Affairs Secretary Francis Tolentino who described the deaths as "casualties."

The official death toll stands at two, according to the national disaster agency.

The city's famed Victoria Harbor was hit with a storm surge of more than 3.5 meters (11.5 feet) Sunday. Hong Kong's iconic skyline, filled with massive buildings jutting up from the hill, was almost completely obscured as squalls roared through, though visibility has improved.

More than 550 flights have been canceled at airports in Hong Kong, Shenzhen and Guangzhou, and more than 200 have been delayed, according to Flightaware.com. Most of Hong Kong's public transport has been suspended.

Hong Kong authorities have been warning residents about the storm for days. On Saturday, grocery stores were packed with people stocking up on goods. Buildings across the city were either boarded up or had their windows taped in order to mitigate the damage of broken glass.

Mangkhut struck the northern Philippines as a super typhoon, killing at last two people and causing flooding and landslides on the northern island of Luzon.

It made landfall in the Philippines Saturday at 1:40 a.m. local time, packing winds of up to 270 kph (165 mph), 120 kph (75 mph) stronger than Hurricane Florence that hit North Carolina.

Known locally as Ompong, Mangkhut ripped roofs off buildings, uprooted trees, blocked roads with debris and dumped water on fields of crops.

More than 250,000 people were affected by the storm across the country, with around half of those seeking shelter in evacuation centers in the country's north.

Philippines President Rodrigo Duterte will head to the region Sunday to see the damage and recovery operations, presidential Palace Communications Secretary Martin Andanar told CNN.

The most severe damage came in Luzon's north, a sparsely populated region that's considered the breadbasket of the Philippines, though areas as far away as Manila -- more than 340 km (200 miles) from the eye of the storm -- were hit with heavy rains that caused flooding in urban areas.

As of Saturday, the storm had caused 51 landslides in the Philippines' north. Search crews are looking for people reported missing in the mountainous Cordillera region, Political Affairs Secretary Francis Tolentino said.

Though the storm system has moved on, extent of the damage has been difficult to assess Sunday as fierce winds were replaced by flood waters, blocking access and aid to affected areas. A vital transportation hub in the region, Tuguegarao airport in northern Luzon, was damaged in the storms, according to the Department of Transportation, forcing the cancellation of more than 100 local and international flights.

Mangkhut is expected to make another landfall late Sunday night, hitting the Chinese province of Guandong near the cities of Yangjiang and Zhanjiang. Other cities in the Pearl River Delta -- which includes, Hong Kong, Guangzhou, Shenzhen, Zhuhai and Macau -- are likely to be impacted.

From there the system will continue to move westward and will rain itself out over northern Vietnam, which could lead to some flooding there early next week.